This invention relates generally to the measurements of fluids flowing through conduits or pipes; and, in particular, to a pressure differential method for the determination of flow rate, fluid velocity and direction in bi-directional low-velocity flow system; and further, to a novel flowmeter in accordance with the said method suitable for the measurement of steady and transient flow, and, especially, applicable to fluid at high-temperature high-pressure conditions.
The flow measurement is accomplished by a variety of means, depending upon quantities, flow rates, and types of fluids involved. Many industrial process flow measurements consist of a combination of two devices: a primary device placed in intimate contact with the fluid and generates a signal, and a secondary device to translate this signal into a motion or a secondary signal for indicating, recording, controlling, or totalizing the conditions of the flow. Other devices indicate or totalize the flow directly through the interaction of the flowing fluid and the measuring device placed directly or indirectly in contact with the fluid stream. A number of flow measurement methods are available; presently, commercialized metering devices can be classified by their operating principles into the following categories: (1) pressure-differential flowmeters; (2) variable-area flowmeters, (3) magnetic flowmeters, (4) turbine flowmeters, (5) oscillatory flowmeters, (6) thermal-loss flowmeters, (7) vortex flowmeters, (8) fluidic-oscillator flowmeters, (9) momentum mass flowmeters, (10) ultrasonic flowwmeters, (11) positive-displacement flowmeters, (12) open-channel flowmeters, (13) laser Doppler velocimeters, and (14) nuclear magnetic resonance flowmeters, and still some others. Such a wide diversity in flowmeter design results on the one hand from multifarious and specific use conditions, and, on the other hand, from tireless progress of science and technology which never a moment stops bringing out wonderful instruments. Nevertheless, to our knowledge, a metering device capable of measuring precisely flow rate and fluid velocity of bi-directional low-velocity flow does not as yet exit.
Conventionally, the commercialized flowmeters are so designed as to suit a flow system driven by pumps. As a consequence, they are not applicable to bi-directional flow systems. Moreover, when a metering device is of the same diameter as the pipeline system being measured, such as the case of turbine or drag disk type flowmeter, its minimum flow range is usually too high to give reliable measurement for low-velocity flow or for natural circulation. The driving force in low-velocity flow system mainly comes from the delicate balance between a very weak gravitational force and the buoyant force of the fluid itself, owing respectively to parts of the flow system assembly positioned at unequal heights, and to density gradient within the flowing stream. In such flow system, it is to be expected that, backward flow may occur when the flow is at transient state. Accordingly, it is the principal objective of this disclosure to overcome the aforesaid problems by providing a novel method and a novel device suitable for precision measurement of bi-directional low-velocity flow. To be the flow at steady or transient state, the accurate data of flow rate, fluid velocity, and direction can be obtained by the extremely sensitive pressure differential metering device of the invention, even if the fluid is at a high-temperature high-pressure condition.
For safety, economy, reliability and other factors, the low-velocity flow precision measurement technique has found many applications in industry, such as the on-line inputs proportion control in chemical continuous processes; such as the cyclic transport and storage of low-velocity fluid within the black pipeline system of a solar energy absorption plate; such as corrosive or toxic liquid wastes disposal, or industrial wastewater disposal, that makes use of a non-power transport system, siphon for instance, such as oil-well drilling and subterranean heat extraction industry where measurement of extracted fluids, including fluid mixtures of different liquid phases (e.g. water/oil) or all kinds of gas-liquid mixtures (e.g. natural gas/hydrocarbons), is massively important; such as natural circulation heat transfer system; and such as the cooling system of an advanced passive nuclear reactor, etc. As to natural circulation of high-temperature high-pressure heat transfer system, the driving force is, as a rule, by far weaker than the forced circulation of a pumping system; thereupon, when the flowing fluid is at transient state, backward flow sometimes occurs. In commercial flow measurement, this backward flow phenomenon is wholly neglected, besides its inability to adequately deal with low-velocity natural circulation. One point worthy of note is that the natural circulation is highly susceptible to flow resistance, therefore it is not advisable to reduce the size of conduit or transport pipeline in accommodation to the installation of a certain commercialized flowmeter of desirable flow range, for that will disturb flow conditions and sometimes even altogether stop the circulation flow.
Among many flow measurement devices as mentioned before, pressure-differential flowmeters embody the oldest method of measuring flowing fluids, and are still most widely in use. In this type of flowmeters, the flow rate is determined from pressure drop (or pressure difference) across a constriction (or restriction) in flow path. They operate on the principle of energy conversion between static pressure and velocity. The velocity increase resulted from a constriction in a pipe will have associated with it a decrease in static pressure. Thus, flow rate can be determined from a measurement of the pressure drop due to constriction. With static pressure-drop data, the fluid mass density and the sectional flow area at the constriction, a theoretical flow rate can be calculated according to Bernoulli""s equation. However, the deviation from this theoretical value is not only possible but prevalent in practice, and this is usually attributed to viscosity change in the fluid passing through constriction, and to the geometry of flowmeter. Traditionally, such deviation is put into consideration in the flow equation by introducing a discharge coefficient which can only be determined by empirical means. More will be said on this later.
The common examples of pressure-differential flowmeters are orifice meter, flow nozzle, and Venturi meter. Orifice meter is a thin plate inserted between pipe flanges, usually having a round, concentric hole with a sharp, square upstream edge. Orifice presents large flow resistance in comparison with the other two devices. In addition, orifice may cause tangible stream turbulence, for, as the streamlines of flow field approaching a sharp-edged orifice they converge on the orifice from all directions, and so as soon as passing through the orifice they continue to converge for a short distance downstream depending on flow rate and fluid viscosity, forming as it were a xe2x80x98free jetxe2x80x99, which contracts to a section somewhat smaller in diameter than the orifice, after which the jet increases in size to fill the pipe. This contracted section of flow field, where the minimum cross-sectional area of said free jet is, is known as vena contracta, which is of supreme importance in the calculation of flow equation and also in the placing of pressure taps. As to flow nozzle, it consists of a bell-shaped approach section of elliptical profile attached to a cylindrical throat tangent to ellipse. Originally, the flow nozzle was designed to reduce pressure loss of fluid flowing through a traditional orifice plate; nowadays it is often used where solids are entrained in the flowing liquid, where the stream is a high-temperature high-pressure flow, and where fluid velocity is high. Only Venturi meter is suitable for low-velocity flow measurement in deed. More than a convergent inlet channel similar to a flow nozzle, Venturi metler has also a divergent outlet channel to reduce turbulent losses, and therefore gives the best measurement accuracy among the known pressure-differential flowmeters.
There is yet again a device of pressure-differential type needs to be briefly mentioned for its being involved in the present invention as will soon be made clear. This is called pitot tube, which is designed to measure the difference between the total impact pressure and the static pressure of a flow system. Pitot tube has a central tube pointing upstream along the direction of pipe axis to receive impact pressure; and the central tube is encased along its wall by an outer tube on whose wall there are pressure holes to measure static pressure. The difference in these pressures, which is called dynamic pressure !differential, can be converted into an indication of the local fluid velocity at the tip of pitot tube. Usually several readings are taken across a pipe at a few locations and the average fluid velocity is determined. This device is an effective tool for laboratory use or for spot checks; only that its tendency to plug when the flowing stream contains small solid particles, its very limited velocity range, and its susceptibility to disturbed flow of varied velocity distribution limit its use in industry.
The present invention combines the advantages of both Venturi meter and pitot tube to make a flowmeter capable of precision measurement in bi-directional low-velocity flow system. An earlier patent disclosure similar to the invention was presented by Brower, William B., Jr., in U.S. patent application Ser. No. 5,365,795 (1994). For comparison as well as for better illustration of the present invention, some explanations and some critical comments are given to this prior example as the follows. FIG. 1 is a graphical representation of a flowmeter design disclosed by Brower. This is a typical Venturi flowmeter, comprising a convergent inlet conical channel 60, a cylindrical throat channel 62, and a divergent outlet conical channel 64, wherein the outlet angle xcex8 is smaller than the inlet angle xcfx86, so arranged as to reduce flow resistance. The total impact pressure P1 and fluid mass density xcfx81 can be derived respectively from fluid velocity and fluid temperature, which are, in this layout, measured by a pitot tube 66 and a temperature probe 68, both devices being inserted into the flow field just a short distance upstream from the convergent inlet conical channel 60. The static pressure P3 is measured by a set of pressure tubes 70 tapped along the central cross section of the cylindrical throat channel 62. With these data one can calculate volumetric flow rate, which is a function of [(P1xe2x88x92P3) /xcfx81]xc2xd. The limitations of this design include; (1) the pitot tube and temperature probe assembly is such an obstruction in the channel that it may cause intolerable flow field turbulence; (2) it can not deal with bi-directional flow; (3) the fluid density data is derived from fluid temperature only, which is inappropriate when phase change such as boiling phenomenon occurs; (4) the fluid density distribution may turn inhomogeneous when the device is horizontally installed; and (5) it can not derive fluid density from available pressure-drop data. As will be seen in the following, the present disclosure is an improvement upon this prior art.
It is the principal objective of this invention to overcome those deficiencies found in commercial flow measurement as described in the foregoing, by providing a novel method of flow measurement and a flowmeter for the determination of flow rate, fluid velocity and direction, in natural circulation and in bi-directional low-velocity flow system, said method and said flowmeter being applicable to steady as well as transient flow, and also to flow at high-temperature high-pressure conditions. The primary device of the flowmeter hereupon disclosed comprises an upstream cylindrical channel, a throat section, and a downstream cylindrical channel. The throat section, similar to la Venturi meter, has three distinct features, namely: a convergent inlet conical channel, a cylindrical middle-channel, and a divergent outlet conical channel. Four sets of static pressure differential data are used to determine flow rate and fluid direction; while fluid velocity is derived from the dynamic pressure differential data measured by a few pairs of impact tubes, similar to pitot tubes, which are inserted within the wall of throat section, the tip of each impact tube protruding a little distance outside the interior surface of either conical channel pointing along the direction of pipe axis. In order to ensure signal stability, and to relieve the work of calibration, each pressure output signal is an average value derived from a few pairs of pressure taps diametrically positioned around the outer wall of said primary device. As this novel metering device is a pressure-differential flowmeter, its accuracy of measurement is evaluated to be 1.5% within its flow range. This flowmeter presents low flow resistance and low flow field disturbance; it is neither restricted by the magnitude of flow rate nor by fluid direction, thereby overcomes the difficulties often encountered in low-velocity flow measurement of large pipeline system, or in cases where backward flow occurs.
According to this disclosure, the flow rate, fluid velocity and direction are obtained with the following procedures. First, static pressure-drop data are measured by several sets of pressure taps along the wall of the primary device 10. Secondly, dynamic pressure-drop data are measured by impact tubes. Thirdly, all the pressure-drop data are averaged and transformed into flow information by a data acquisition system of a microprocessor with build-in calibration curves and capable of on-line fluid density correction. One special feature of this invention is that it does not make use of the empirical fluid discharge coefficient in the calculation of flow rate. The value of discharge coefficient is influenced by many factors which fall into two general categories: (1) dynamic factors involving the properties of the flow, such as viscosity, fluid mass density, etc; and (2) geometric factors involving the geometry of meter installation and the constructional features of the pipeline, such as the ratio of throat diameter to pipe diameter, the convergent and divergent angles of the conical channels as in the case of a Venturi meter, roughness of interior surface of the pipeline system as well as of the metering device, the size of the pipe, and the locations of pressure taps etc. With the help of a micro-processor stored with calibration data and capable of on-line fluid density correction, the present invention obviates many complicating and some still uncertain experimental procedures needed to determine the value of fluid discharge coefficient, thereby proves more efficient than other pressure-differential flowmeters.